Hybrid Brayton thermosolar systems: thermodynamic prediction of annual efficiencies and emissions

[EN]The necessity to diversify the energy sources in power generation and to look for renewable ones is undoubted. Thermosolar power plants, which constitute one of the main ways of solar energy exploitation, are competing with other renewable energy sources for generating clean electrical energy, reducing fuel consumption. Hybrid thermosolar plants combine two great advantages on electricity generation: the emissions reduction of thermosolar energy, as well as the stable supply of power output to the grid of conventional power plants, avoiding the use of storage systems. For those reasons in the last years a big effort has been done in the development of prototypes and experimental plants in order to investigate the viability of thermosolar hybrid Brayton cycle plants. A working fluid, usually air, is preheated by concentration solar energy, before entering a combustion chamber. Then, the fluid performs a thermodynamic cycle (in this case, a Brayton cycle), generating electrical energy indirectly. In this way fossil fuel and the associated emissions are reduced. It is important to note that apart from being easily scalable, gas-turbines can be combined with other cycles like bottoming Rankine. Also they do not require too much water for operation, which makes them suitable for electrical generation in arid regions, and are extremely versatile [1]. Experimental projects and prototypes developed up to date show that this technology is viable, but they also reveal that it is necessary to improve their efficiency, in order to generate electricity at competitive prices. Apart from R+D projects, prototypes, and experimental installations, several research works have been published in the last times. Some of them make use of commercial simulation environments, which allow a detailed description of all plant components and specific calculations on the solar subsystem. However, it is not easy to extract direct physical information about the main losses sources in the plant and to perform a global optimization of the plant design. Because of this reason, in this paper the next modus operandi is followed instead of this one. A second type of strategy is to build a theoretical model of the plant, in terms of a reduced number of parameters, allowing a simple but realistic picture of plant operation and to estimate its performance records. Thermodynamic analyses can provide an integrated point of view of all subsystems and their importance in the overall efficiency. Moreover, they help to predesign future generations of plants based in this concept because of their flexibility to survey the adequate intervals of key parameters for optimal plant operation. There are several theoretical works that start from the ideal Brayton cycle and thereafter refinements are included in the analysis of the thermodynamics of the cycle in order to recover realistic output records. Usually, in these works, the model for the concentrated solar subsystem, although including the main heat transfer losses, is simple. This allows to obtain closed analytical expressions for thermal efficiencies and power output, and then check the model predictions for particular design point conditions, with fixed values of direct solar irradiance and ambient temperature. But also by means of this thermodynamic model, a dynamic analysis that varies solar irradiance and external temperature conditions with time can be carried out. And in a possible step forward to suggest and guide optimization strategies